1. Field of the Invention
The present invention relates to wet oxidation processes and apparatus. More particularly, this invention relates to apparatus which utilize zirconium oxide ceramics for surfaces exposed to high temperature water oxidation environments, and to water oxidation processes conducted therein.
2. Description of the Related Art
Wet oxidation is a process that involves the addition of an oxidizing agent, generally air or oxygen, to an aqueous stream, at temperatures and pressures sufficient to cause the "combustion" of oxidizable materials directly within the aqueous phase. Typical wet oxidation temperatures and pressures are generally in the range of about 150.degree. C. to about 370.degree. C. and in the range of about 30 to about 250 bar (3626 psia). Wet oxidation has been used for the treatment of aqueous streams for many years. For example, U.S. Pat. No. 2,665,249, issued Jan. 5, 1954 to Zimmermann discloses oxidation in the aqueous phase of carbonaceous dispersions.
Wet oxidation as such is limited by the degree of oxidation achievable, an inability to adequately handle refractory compounds, slow reaction times, and lack of usefulness for power recovery due to the low temperature of the process. Much development in the process of wet oxidation has been in the direction of increasing the operating temperatures and pressures.
In an effort to overcome some of the prior art limitations, U.S. Pat. No. 2,944,396, issued July 12, 1960 to Barton et al. disclosed the addition of a second wet oxidation stage. The unoxidized volatile combustibles which accumulate in the vapor phase of the first stage are sent to complete their oxidation in the second stage, which is operated at temperatures above the critical temperature of water of about 374.degree. C. U.S. Pat. No. 4,292,953, issued Oct. 6, 1981 to Dickinson, disclosed a modified wet oxidation process for power generation from coal and other fuels in which, as heat is liberated by combustion, the entire reaction mixture exceeds the critical temperature of water of about 374.degree. C., with operative pressures of about 69 bar (1000 psi) to about 690 bar (10,000 psi) spanning both the sub- and supercritical water pressure ranges. U.S. Pat. No. 4,338,199 issued Jul. 6, 1982, to Modell, disclosed a wet oxidation process, sometimes known as supercritical water oxidation, in which the oxidation is initiated and carried out at supercritical temperatures (above about 374.degree. C.) and pressures (above about 3200 psi or about 220 bar). With supercritical water oxidation (SCWO), almost any compound can be substantially completely oxidized in a matter of seconds, with destruction efficiencies on the order of 99.9999% easily obtainable. As a result, supercritical water oxidation is contemplated for the destruction of hazardous and toxic wastes, as an alternative to incineration.
For wet oxidation at temperatures below about 300.degree. C. titanium metal has proven to be resistant to a wide spectrum of environments, and is an acceptable material of construction for such processes. Furthermore, stainless steel has proven suitable for high temperature liquid oxidation research in dealing with mixtures of water, oxygen and hydrocarbons, but it is anticipated that stainless steel will be inadequate Furthermore, stainless steel has proven suitable for high temperature liquid oxidation research in dealing with mixtures of water, oxygen and hydrocarbons, but it is anticipated that stainless steel will be inadequate for commercial systems which will have a variety of acidic, alkaline, and salty streams. The various processes for oxidation in an aqueous matrix will hereinafter be referred to collectively as high temperature water oxidation (HTWO) if carried out at temperatures above about 300.degree. C., and at pressures generally in the range of about 27.5 bar (400 psi) to about 690 bar (10,000 psi). In high temperature water oxidation of toxic and hazardous wastes the unique operating conditions and chemical environment greatly limit the selection of materials of construction. This environment is described below for the particular case of SCWO, though other HTWO environments will have much in common.
As for describing the chemical environment, the products of complete oxidation in supercritical water oxidation are fairly well known. Carbon and hydrogen form the conventional combustion products CO.sub.2 and H.sub.2 O. Chlorinated hydrocarbons also give rise to HCl, which will react with available cations to form chloride salts. Alkali may be intentionally added to the reactor to avoid high concentrations of hydrochloric acid. In contrast to normal combustion, which forms SO.sub.2, the final product of sulfur oxidation in SCWO is sulfate anion. As in the case of chloride, alkali may be intentionally added to avoid high concentrations of sulfuric acid. Similarly, the product of phosphorus oxidation is phosphate anion.
While it is frequently desirable to neutralize oxidation product anions via alkali addition, the reverse is not usually true. Feedstocks containing excess noncombustible cations are generally self-neutralized by the CO.sub.2 evolved from oxidation. For example, a stream containing organic sodium salts will yield sodium carbonate or bicarbonate as a product. Ammonium, another common cation, can be converted to water and dinitrogen (N.sub.2) or nitrous oxide (N.sub.2 O) in the SCWO process, and so may not require neutralization.
A key advantage of SCWO over incineration is the lack of NO.sub.x formation due to the relatively low temperature of operation. Oxidized forms of nitrogen, e.g., organic nitro-compounds and nitrate anion, have been found to form N.sub.2 or N.sub.2 O to a greater or lesser degree, just as in the case of ammonia and other reduced forms such as proteins. When air is used as the process oxidant, the N.sub.2 passes through the system as an inert.
The chemical environment of supercritical water oxidation when used for waste processing will frequently include mineral acids HCl, H.sub.2 SO.sub.4, HNO.sub.3, and H.sub.3 PO.sub.4, the alkaline materials NaOH and Na.sub.2 CO.sub.3, and various salts with cations such as Na, K, Ca, and Mg.
While the chemical components present in SCWO reactions are well documented, much remains to be learned about chemical kinetics and reaction mechanisms. The situation is complicated by the wide range of densities which can exist in supercritical water systems. At the typical reactor conditions the supercritical phase density is on the order of 0.1 g/cc. Reaction mechanisms are of the free radical type, as with normal combustion, through greatly affected by the much higher density and water concentration. On the other hand, at temperatures closer to the critical point, or in dense brine phases, densities of 0.5 to 1 g/cc and higher are obtained and ionic reaction mechanisms will dominate. Similarly, corrosion mechanisms will differ depending on operating conditions.
The corrosivity of a particular chemical is partly dependent upon its phase state. At typical SCWO reactor conditions with densities in the range of 0.1 g/cc, water molecules are considerably farther apart than in normal liquid water. Hydrogen bonding, a short-range phenomenon, has been almost entirely disrupted, and the water molecules lose the ordering responsible for many of liquid water's characteristic properties. In particular, solubility behavior is closer to that of high pressure steam than to liquid water. Smaller polar and nonpolar organic compounds, with relatively high volatility, will exist as vapors at typical SCWO conditions and hence be completely miscible with supercritical water. Gases such as N.sub.2, O.sub.2, and CO.sub.2 show similar complete miscibility. Larger organic compounds and polymers will largely pyrolyze to smaller molecules at typical SCWO conditions, resulting in solubilization via chemical reaction. The loss of bulk polarity by the water phase has striking effects on normally water-soluble salts, as well. No longer readily solvated by water molecules, they precipitate out as solids or dense brines. The small salt residual in the supercritical phase is largely present in molecular form, e.g., as NaCl molecules. Heavy metal oxides, of low solubility in liquid water, retain their low solubility at supercritical water conditions. Exceptions exist and high solubilities occur, however, when a metal can form a volatile salt, oxide, or elemental compound at reactor temperatures.
The preceding characterization of solubility behavior has been given in relation to pure supercritical water. In actual SCWO systems, this behavior can be greatly altered by the presence of large quantities of gases and salts. In many applications, for example, the mass of "noncondensible" gases such as N.sub.2 and O.sub.2 in the reactor may exceed the mass of water present. The presence of noncondensible gases and salts in the SCWO reactor encourages the separation of phases, similar to the familiar phenomenon of "salting out" of gases from solution.
The combination of highly oxidizing conditions, acid gases, and caustic or high salt solids and dense brines described here is an extremely aggressive chemical environment. In addition to the harsh chemical environment, the operating conditions in HTWO are also very demanding. Any materials utilized in a high temperature liquid oxidation system will have to withstand temperatures and pressures of at least about 300.degree. C. and about 27.5 bar (400 psi), and in some instances even ranging up to and exceeding 600.degree. C. and 690 bar (10,000 psi). The materials must also withstand thermal shock which may be imposed intentionally or under upset conditions.
Although high nickel alloys such as Hastelloy C-276 or Inconel Alloy 625 have been suggested for use as materials of construction for high temperature wet oxidation reactors (see U.S. Pat. No. 4,543,190), test data indicate that such materials exhibit unacceptably high corrosion rates at reactor conditions. Furthermore, prolonged exposure at and cycling of these materials to reactor temperatures leads to a degradation of their mechanical properties. Both alloys are subject to embrittlement, giving rise to the possibility of cracking and catastrophic failure. A large number of other metals and alloys have been tested with the hope of finding one suitable. Nevertheless, with the possible exception of certain noble metals too expensive for general usage, it now appears that no metal or alloy exists which has satisfactory corrosion resistance to commercially envisioned HTWO environments. Firebrick has also been suggested as a material of construction in large diameter SCWO reactors. Due to relatively high solubility, however, the alumina/silica composition of firebrick is unsuitable for many HTWO environments, especially when caustic materials such as NaOH and Na.sub.2 CO.sub.3 are present.
Stubican et al., suggested in Science and Technology of Zirconia (1981) that stabilized zirconia ceramics might have application as heat-resistant linings in furnaces and as protective coatings on alloys. However, Stubican et al., did not disclose or suggest that such stabilized zirconia ceramics would be able to face the harsh environment of high temperature water oxidation. Furthermore, Swab, in Low Temperature Degradation of Y-TZP Materials (U.S Army Materials Technology Laboratory Report No. MTL TR 90-4, January 1990) while investigating the suitability of yttria stabilized zirconia for use in heat engines, discloses that such materials are susceptible to reactions with water vapor at temperatures from 200.degree. C. to 400.degree. C. This finding would suggest that such stabilized zirconia materials would be unsuitable for high temperature water oxidation.
Because of the unsuitability of metals and conventional ceramics for use as materials of construction for water oxidation systems in which temperatures above 300.degree. C. are encountered, a need exists for a resistant material for use in constructing such systems.